1. Field of the Invention
Example embodiments of the present invention relate generally to an apparatus and methods for capturing a fingerprint, and more particularly to an apparatus and methods for capturing a fingerprint based on information extracted from a guard region of a captured image.
2. Description of the Related Art
A fingerprint is a distinctive characteristic of an individual. Generally, each person may have a different fingerprint pattern which typically does not change during the life of the person. Accordingly, fingerprints may be a useful tool for identification. For example, conventional security systems, such as financial transaction systems, may deploy fingerprint scanners connected to fingerprint recognition systems for identifying and authenticating people seeking access to protected services.
A conventional fingerprint recognition system may store a fingerprint image of an individual obtained via a fingerprint acquisition apparatus (e.g., a fingerprint scanner). The conventional fingerprint recognition system may perform fingerprint recognition by comparing the fingerprint image with a stored fingerprint. During the comparison, the fingerprint recognition system may extract fingerprint characteristics from the obtained fingerprint image and may compare the extracted fingerprint characteristic with a fingerprint characteristic of the stored fingerprint image.
Conventionally, a process of extracting a fingerprint characteristic from a fingerprint image may include extracting a fingerprint region from the fingerprint image, extracting a ridge direction and a frequency from the fingerprint region, enhancing the quality of the fingerprint image, and binarizing and thinning the fingerprint image with the enhanced quality. For example, the process of extracting the fingerprint region may be performed to separate the fingerprint region from a background region. The ridge direction of the fingerprint may be obtained by extracting the direction of a ridge. The ridge direction may include information used in an overall fingerprint recognition algorithm. For example, a conventional fingerprint recognition algorithm may include a process of searching for singular points (e.g., core, delta, etc.), a ridge enhancement process, and a classification process. A gradient of a fingerprint image may also be used to aid in the ridge direction extraction.
A fingerprint image may be divided into a plurality of non-overlapping blocks. FIG. 1A illustrates a conventional block of a fingerprint image. The gradient (Gx, Gy) of each block (e.g., such as illustrated in FIG. 1A) may be obtained by using a Sobel operator. The ridge direction for each block may also be obtained by using the gradient (Gx, Gy) by applying a series of calculations, described below with respect to Equations 1 through 6:
                              G          x                =                              ∇            x                    ⁢                      (                                                            x                  i                                +                h                            ,                                                y                  j                                +                k                                      )                                              Equation        ⁢                                  ⁢        1                                          G          y                =                              ∇            y                    ⁢                      (                                                            x                  i                                +                h                            ,                                                y                  j                                +                k                                      )                                              Equation        ⁢                                  ⁢        2                                          G          xx                =                              ∑                          h              =                              -                w                                      w                    ⁢                                    ∑                              k                =                                  -                  w                                            w                        ⁢                                          ∇                x                            ⁢                                                (                                                                                    x                        i                                            +                      h                                        ,                                                                  y                        j                                            +                      k                                                        )                                2                                                                        Equation        ⁢                                  ⁢        3                                          G          yy                =                              ∑                          h              =                              -                w                                      w                    ⁢                                    ∑                              k                =                                  -                  w                                            w                        ⁢                                          ∇                y                            ⁢                                                (                                                                                    x                        i                                            +                      h                                        ,                                                                  y                        j                                            +                      k                                                        )                                2                                                                        Equation        ⁢                                  ⁢        4                                          G          xy                =                              ∑                          h              =                              -                w                                      w                    ⁢                                    ∑                              k                =                                  -                  w                                            w                        ⁢                                                            ∇                  x                                ⁢                                  (                                                                                    x                        i                                            +                      h                                        ,                                                                  y                        j                                            +                      k                                                        )                                            ⁢                                                ∇                  y                                ⁢                                  (                                                                                    x                        i                                            +                      h                                        ,                                                                  y                        j                                            +                      k                                                        )                                                                                        Equation        ⁢                                  ⁢        5                                          θ          ij                =                              90            ⁢            °                    +                                    1              2                        ⁢                          arctan              ⁡                              [                                                      2                    ⁢                                          G                      xy                                                                                                  G                      xx                                        -                                          G                      yy                                                                      ]                                                                        Equation        ⁢                                  ⁢        6            wherein (xi, yi) may denote the coordinates of an (i, j)-th pixel in the (x, y) direction in the fingerprint region, a width and a length of each block may each be 2w, θij may denote a ridge direction in a block having a central pixel with coordinates (xi, yi).
FIG. 1B illustrates pixels positioned within a gradient (Gx, Gy) for the conventional block of FIG. 1A. The straight line in FIG. 1B may indicate the ridge direction in the block. Referring to FIG. 1B, the ridge direction may be represented as a straight line diagonally crossing the gradient (Gx, Gy). The ridge direction illustrated in FIG. 1B may be based on the pixels calculated with Equations 1 through 6 as discussed above.
A maximum direction in a gradient field may be determined as the ridge direction in the block using a least square (LS) algorithm. However, determining the maximum direction in this manner may increase an affect of outliers. For example, a wound at the finger and/or noise in the fingerprint may affect the maximum direction, thereby reducing the relevance of the calculated maximum direction.
FIG. 2 illustrates a ridge direction attained via a conventional least square (LS) algorithm. As shown in FIG. 2, due to outliers present within the gradient, the ridge direction may be disproportionately skewed by the outliers such that an inaccurate ridge direction (θtrue) may be obtained instead of an accurate ridge direction (θLS).
Cameras may be mounted in portable terminal apparatuses (e.g., mobile phones, PDAs, etc.). The development of more accurate image sensors and image processing technologies capable of deployment within smaller-scale devices may facilitate higher quality image extraction from portable terminal apparatuses. Accordingly, biometric authentication, such as fingerprint recognition, may be implemented at such portable terminal apparatuses based on fingerprint images obtained through a camera mounted thereon. However, fingerprint images obtained through digital image capture at portable terminal apparatuses may be less accurate than fingerprint images obtained with conventional contact-type fingerprint acquisition apparatus, such as a fingerprint sensor. Accordingly, it may be difficult to extract a fingerprint image and the direction of a fingerprint ridge from a fingerprint image obtained through a camera mounted on a conventional portable terminal apparatus.
FIGS. 3A and 3B illustrate fingerprint images obtained with a conventional contact-type fingerprint sensor. FIG. 3C illustrates a fingerprint image obtained with a camera mounted on a conventional portable terminal apparatus.
As shown in FIGS. 3A and 3B, the conventional contact-type fingerprint sensor, a background part may appear uniformly. The background part of the fingerprint image may refer to a portion where a user's finger may not be in contact with the contact-type fingerprint sensor during the fingerprint image scan. Accordingly, values such as mean brightness and/or variance of the fingerprint image may allow the fingerprint region and the background region to be distinguished.
However, as shown in FIG. 3C, the background part of a fingerprint image obtained with the camera of a portable terminal apparatus may be cluttered with objects peripheral to the user's finger. Accordingly, it may become more difficult to distinguish between the background part and the fingerprint region in fingerprint images obtained with a camera of a portable terminal apparatus. Further, an illumination difference between the ridge and valley of a fingerprint image obtained with a camera of a portable terminal apparatus may be lower than that obtained with the conventional contact-type fingerprint sensor. For example, a wound of a finger may more negatively affect fingerprint recognition of fingerprint images obtained with a camera (e.g., mounted on a portable terminal apparatus) than a conventional contact-type fingerprint sensor.